Specificiteten

Specificiteten, also called specificity in English, is a measure used to evaluate diagnostic tests. It is defined as the proportion of true negatives among all individuals who do not have the target condition, reflecting the test’s ability to correctly identify negatives and to avoid false positives. Specificiteten is independent of disease prevalence in the study population.

Calculation is straightforward: specificiteten = true negatives / (true negatives + false positives). In rate form, it is equivalent to 1 minus the false positive rate, where the false positive rate equals false positives divided by (false positives plus true negatives).

Specificiteten sits alongside sensitivity, which is the true positive rate. Tests often involve a trade-off between sensitivity and specificity depending on the chosen threshold or cut-off. A high threshold may increase specificity at the cost of sensitivity, while a lower threshold can do the opposite. Receiver operating characteristic (ROC) curves illustrate this balance, with the area under the curve providing an overall measure of discrimination.

Applications and limitations: Specificiteten is a key criterion for screening and diagnostic tests, guiding decisions about follow-up testing and treatment. However, it depends on the selected reference standard and the characteristics of the study population. Factors such as spectrum bias, misclassification of disease status, and sample selection can affect reported specificity. Unlike sensitivity, specificity does not depend on disease prevalence, but predictive values (positive and negative) do depend on prevalence.